(1) Field of the Invention:
The present invention relates to a process for recovering nitric acid directly from waste nitric acid or a nitrate, and an apparatus for practising this process. More particularly, the invention relates to a process for recovery of nitric acid comprising incorporating or absorbing waste nitric acid and/or a nitrate in an oil-absorbing inorganic carrier and forming a nitrate-carrier granule containing water in the form of a hydroxide, a hydrate or adsorption moisture in an amount sufficient to generate a vapor of nitric acid stably and being substantially non-sticky at a decomposition temperature of the nitrate; heating the formed granule while keeping the substantially non-sticky state in the granule to decompose the nitrate in the granule to a vapor of nitric acid; and recovering the generated vapor of nitric acid.
(2) Description of the Prior Art:
In various chemical and metal industries, mineral acids such as sulfuric acid are used in large quantities for various treatments. For example, a so-called sulfuric acid is known as an industrial process for the production of titanium oxide, but in such process using sulfuric acid, many problems are left unsolved in connection with prevention of environmental pollution. For example, it is said that in the sulfuric acid process for the production of titanium oxide, about 8 tons of dilute sulfuric acid (including washing water) and 3 to 4 tons of hydrous iron oxide are produced as by-products for production of 1 ton of titanium oxide. However, from the viewpoint of prevention of environmental pollution, it is not allowed to discard such dilute sulfuric acid in a river or sea, and such dilute sulfuric acid should be treated again in a particular treatment plant. It is said that the cost of the product is increased by about 15% because of this additional treatment. This disadvantage is also caused when various clay minerals or other mineral substances are acid-treated or iron or steel is washed with an acid.
Among various mineral acids, nitric acid can be distilled at a relatively low temperature and a nitrate can be thermally decomposed at a relatively low temperature. Accordingly, nitric acid can easily be recovered from waste nitric acid or a nitrate, and it may be considered that by utilizing this characteristic property of nitric acid, nitric acid will be used for various acid treatments. However, various problems must be solved when nitric acid is recovered from waste nitric acid or a nitrate.
As one of such problems, there can be mentioned the disposal of a nitrogen oxide gas, so-called NO.sub.x gas, formed at thermal decomposition of a nitrate or at distillation of waste nitric acid. This NO.sub.x gas is not satisfactorily absorbed in an absorbing liquor such as water, and in order to recover such NO.sub.x gas in the form of nitric acid, the NO.sub.x gas must be passed through a column packed with an oxidation catalyst to convert nitrogen monoxide to nitrogen dioxide, and for this treatment, the equipment must be enlarged and the treatment cost becomes high.
Another problem is adhesion of a nitrate which is molten at a relatively low temperature at the thermal decomposition of the nitrate, to the wall of a reaction vessel. This adhesion results in corrosion of the wall of the reaction vessel, and it is difficult to perform the thermal decomposition treatment of the nitrate in a continuous manner.
Waste nitric acid recovered from various acid treatments such as mentioned above contains dilute free nitric acid and a nitrate. In order to recover nitric acid in the reusable form from such waste nitric acid, it is necessary to perform troublesome steps of concentrating waste nitric acid to separate a part of free nitric acid in the form of the NO.sub.x gas and thermally decomposing the nitrate-containing residue. At the step of the thermal decomposition of the nitrate, it is molten prior to or on decomposition, and the nitric acid component is recovered in the form of the NO.sub.x gas. Accordingly, occurrence of the above-mentioned disadvantages cannot be avoided.